The electronic structure and the optical properties associated with antisite defects in cubic SiC have been computed by means of the LMTO (linear muffin-tin orbital) method and the supercell approach. The orbital-dependent LDA + U potential (LDA ≡ local density approximation) used in the present work gives rise to an improved description both of the electronic structure near the energy gap and of the optical functions. Attention has been mainly focused on the effects caused by the local lattice relaxation around the defects. For compositions that deviate from the stoichiometric SiC towards higher content of carbon atoms, the small reduction of the energy gap which is observed experimentally can be explained only if the lattice relaxation is taken into account. The local electronic structure of antisite defects is characterized by s-and p-like resonance states in the valence band. Strong resonances occur also in the conduction band (especially for C Si ). The Si C (C Si ) antisite has more (fewer) valence electrons localized in the atomic sphere than the official Si (C) atom, but this difference is considerably reduced by the lattice relaxation. The results of the calculations show how the presence of point defects modifies the shape of the optical functions of the perfect SiC crystal and how the lattice relaxation has a strong effect on the fine structure of the optical functions. Different kinds of defect lead to different shapes of the optical functions.

The local electronic structure around vacancies and vacancy-antisite complexes in cubic SiC has been calculated by means of the LMTO (linear muffin-tin orbital) method and the supercell approach. In order to improve the description of the electronic structure near the energy gap, the orbital-dependent LDA+U potential has been used. Different models for the displacement of the atoms around the defects are discussed. It has been shown that the atoms surrounding a silicon vacancy (VSi ) relax outwards from the vacancy, while for a carbon vacancy (VC ) a distortion of the tetrahedral arrangement of atoms is the more reliable model. The calculations of the local electronic structure performed for the vacancy-antisite complexes show that the VSi + SiC complex is characterized by a repulsive interaction between the SiC antisite and the three carbon atoms surrounding the vacancy. In contrast, the stability of the VC + CSi complex is determined by the bonding of the atoms surrounding the vacancy. In all cases the appearance of the localized electron state in the middle region of the energy gap is a characteristic feature of the vacancy-antisite complex. Some possible mechanisms of the annealing out of the vacancies in -SiC are discussed.

The electronic structure of four SiC polytypes has been computed by means of the LMTO (ASA+CC) method. It is shown that the results of the calculations are very sensitive to the choice of the atomic sphere radii, if the combined corrections are not taken into account in the ASA. The local electronic structure shows a different behaviour between the states in the cubic bilayers and those in the hexagonal ones. It is found that the interaction of c and h bilayers plays a major role, giving rise to changes in the polarity of the Si-C bonds and to charging of bilayers. The effect of the lattice distorsion on the local valence electron charge has been investigated and discussed.